It's quite easy to get unique hair colors by using reagents. I've found a few ways to get different unique hair, eye, scale, skin and fur colors.
Will add on as I experiment with more transformatives not documented in the wiki.
I mostly made this for myself. I didn't include all the possible combinations, but here is a list of the notable ones:
HAIR
Orange, Ruby, Pink, Crimson - Ashen Leaf & Cinderleaf
Indigo, Light Blue, White, Sapphire - Frostleaf
Black, Blue, Red, White, Green - Gaia Leaflets (Drops from Effigies. Bought from Kohaku and Tamarind.)
Black, Blue, Gray, Light-Blue, Pink, Red - Ice Rose (You can find them exploring the Frostwood and Glacial Rift, and also from Senja and Tamarind after Act I.)
Black, Brown, Gray - Ursus Bulb (Randomly spawns exploring the Old Forest.)
Blonde, Blue, Silver - Wyld Wine (Sold by Ivris and Tamarind.)
Black, Brown, Red - Brute Beet (Sold by Ivris and Pavo.)
White, Red, Pink, Brown - Glitter Dust (Dropped by Lusina. Purchasable from Kohaku, Senja, and Viviane.)
Blonde - Hornet Sting (Find in the Old Forest, spawns randomly. Can also find from Farrah. Described as being flaxen in color.)
Dirty Blonde - Bimbo Brew (Dropped by Demons, Imps and purchased from Farrah.)
SCALES
Red, Ruby, Pink, Crimson - Ashen Leaf (Nash sells these at the Minotown Outskirts.)
Crimson, Red, Ruby - Cinderleaf (Purchasable from Kohaku, Senja, Vivi.)
Blue, Indigo, Sapphire - Frostleaf (See above.)
Black, Brown, Orange, Green, Red, White - Scale Salve (Depends on gender preference. Bought from Kohaku at the Kitsune Den or Senja at Oxana's Mansion.)
Black, Dark Green, Dark Red - Manticore Nip (Drops from Manticores, purchased from Kohaku and Tam.)
Ocean Blue - Siorc Bulbs (Purchased from Imora at the Glacial Rift.)
EYES
Gold, Red - Ashen Leaf
Red - Blackened Herb (Bought from Nash.)
Black & Gold - Cinderleaf, Ashen Leaf, Frostleaf
Luminescent Purple, Luminescent Green - Gaia Leaflets
Black - Hornet Sting
Blue, Green, Hazel - Ice Rose (I think these colors are bugged and are supposed to be different if I'm not mistaken?)
SKIN
Brown, Black, Olive, Tan, White - Ashen Leaf
Dark Green, Dark Red, Gray, Light Green - Brute Beet
Bronze, Ebony, Onyx - Cinderleaf
Cream, Light Blue, Pale, White - Frostleaf
Caramel - Hornet Sting
Pink - Pink Truffle (Anna at Pupperidge Farm and Tamarind has it.)
Blue, Brown, Green - Gaia Leaflets
Dark Green, Green, Green Yellow, Light Green - Grottato (Spawns randomly in Frostwood. Also purchasable from Viviane, Senja, and Kohaku.)
Blue, Pink, Purple, Red - Succubus Milk & Incubus Cum (Dropped by demons and imps. Farrah at the Wayfort also sells these.)
Dusky, Pale - Wyld Wine
Blue, Lavender, Light Pink, Purple - Ice Rose
FUR
Black and Brown Striped, Black and Gray Spotted - Cackle Berry (Spawns randomly at the Old Forest. Dropped by Gnolls. Purchasable from Kavi at Marefolk Village and Kohaku.)
Black, Brown, Orange, White - Catnip Tea (Sold by Senja.)
Gray - Leothran Herb (Purchasable from Vari in Frostwood.)
Black, Brown, White - Ursus Bulb
Black, Dark Brown, Light Brown - Wild Chicory (Spawns exploring the Old Forest. Purchasable from Ivris and Senja.)
Black, Blonde, Orange, Red, White - Fox Berry (Randomly spawns in the Old Forest and Frostwood. Purchasable from Senja, Kohaku, and Viviane. Dropped by Kitsune. Pretty easy to come by.)
Red, Black, Gray, Silver - Lycanthorn. (Found exploring the Old Forest. Purchasable from Farrah and dropped by Corrupted Lupines.)
These are all I could find with reliable information. I usually prefer this route if the color is unique.